Yahoo’s global email service is experiencing a fault making it impossible for users to send and receive messages or make use of their webmail accounts.
In the UK, the problem has also affected BT, Sky and TalkTalk’s email accounts, which are powered by the US brand’s service.
Downdetector indicates that the problem began at about 07:00 BST.
Yahoo says it is “working diligently” to solve the problem.
The business is owned by the US communications firm Verizon.
“Yahoo is currently experiencing a global outage, which means that many of our customers are unable to access their BT Yahoo email accounts,” said a spokesman for BT.
“Yahoo is working to fix the issue as quickly as possible, we’re sorry for any inconvenience caused.”
TalkTalk and Sky have also tweeted about the matter.
Those with AOL accounts have also been affected.
Internet faults of one kind or another are not uncommon, but it is relatively unusual for them to last more than six hours, as has happened in this case.
In the past half hour, some customers have reported being able to access their accounts again, but say that several hours-worth of emails appear to be missing.
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